(1) Field of the invention
The invention relates to a transmission system for transmitting binary data symbols in synchronous consecutive symbol time intervals having a duration of T seconds.
The invention is particularly applicable to the field of data transmission using the cables of the public telephone system, between the subscribers and the telephone exchanges, the spectrum of the binary information signal extending from 0 Hz to a certain highest frequency. The component at 0 Hz, the d.c. component, will be absent from the transmitted signal when transformer couplings are present in the cable.
(2) Description of the prior art
A generally known method of transmitting binary information signals without a d.c. component and with sufficient clock information to enable a clock signal to be easily generated is the bi-phase modulation. In accordance with this method the binary information signal is phase modulated on a carrier at the frequency which equals the bit rate (1/T Hz), which is in-phase with the information signal. U.S. Pat. No. 3,846,583 describes a variant of this method. The phase of the carrier according to this variant is shifted through 90.degree. which respect to the binary information signal. In view of the special pulse shape then occurring the name "top hat" modulation or code was proposed for this variant. The amplitude spectra of the modulated signals in accordance with these modulation methods are shown in FIG. 3 by means of the curve BP for the bi-phase modulation and by means of the curve TH for the "top hat" modulation. These two spectra are asymmetrical with respect to the frequency which equals the bit rate (fT=1).
In the above methods, for an optimum detection of the symbols without intersymbol interference, a receive filter will be required whose filter characteristic has an opposite asymmetry. However, this results in a detection which is not so optimum as regards the signal-to-noise ratio.